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Evolution of the sexes Men are evolving to have healthier genes thanks to their genetic relationships with women, new research suggests.

Although, say the researchers, the effect of culture could well be drowning out this genetic signal.

The research, by Dr Stephen Stearns, of Yale University, and colleagues, reveals that genetic connections between males and females strongly affect each others' responses to natural selection.

"Because those connections exist, men respond differently evolutionarily to natural selection than they would if they were not connected to females. In this case they respond in a way that improves their health," says Stearns.

"Males are under selection to get healthier - to have lower total cholesterol, lower blood glucose and lower blood pressure because of the effects coming through females, not because of effects acting directly on them."

Genes that influence bodily traits are under different selection pressures in women and men.

So what impact do these different pressures have on traits given that, over the generations, genes spend roughly half their time in males, and half their time in females?

To explore this, Stearns and colleagues analysed a number of traits in 1500 family trees, containing 15,000 people taken from a long term medical study in the United States.

Their study, reported today in the journal Proceedings of the Royal Society B looked at traits including height, weight, blood pressure and glucose, total cholesterol, and age at first birth, all of which are affected by our genes.

First the researchers measured how each trait affected the number of offspring men and women had. This showed the degree of evolutionary advantage each trait gave individual people.

For example, women with low cholesterol levels, lower blood pressure and lower blood glucose, had more children. But the number of offspring had by males was not affected by these traits.

Next, Stearn and colleagues looked at how the whole network of traits interacted and actually played out in the family trees.

They found the frequency of 'healthy' genes was higher in men related to women with these genes.

Stearns says the findings, supported by other unpublished research he has done, show that men are evolving towards having lower cholesterol, lower blood pressure and lower blood glucose, despite there being no apparent selective advantage to them.

"The main threat factors for heart disease and diabetes are all being selected for in males in a positive direction," he says.

The study also found men also influenced women's evolution, but the effect was not as strong, says Stearn.

Stearn says the study does not shed any light on the mechanism by which the genetic connections between men and women leads to these outcomes.

The culture effect

"It's wonderful," he says. "The novel thing about the paper is it's done on humans."

"They've tapped into a wonderful long-term medical study that's got the pedigree that allows you to get into the genetic information."

Brooks says culture can add an enormous layer of complexity to such studies.

Stearns says he and colleagues tried as much as they could to control for the influence of culture in the study.

But, he says, culture could still be affecting the outcome of the study.

Certainly, cultural factors such as diet and exercise may well be cancelling out the effect any genetic trend towards healthier genes in men.

"It is profoundly important that culture is changing rapidly," says Stearns. "Culture is actually changing the selection pressures on biological traits and that is causing an increasing divergence between culture and biology."